By P. G. C. BRETT The conclusion of the tenth successive crossing
season seems an opportune time to review the present status of sugarcane breeding in Natal. In doing so, the methods of the present will be considered in relation to the results of the past and. the aims of the future. The opportunity will also be taken of considering possible changes in the present varietv position.
Sugarcane Brooding in Natal f'rom 1944 to 1953 Since; the first seedlings of commercial types of sugarcane were raised in. 1944, steady progress has been made in extending the scope of sugarcane breeding in Natal. The first seedlings were obtained by collecting and sowing the seed of tassels which had matured naturally in the field.1 Only thirty-six seedlings were obtained in this way, but nevertheless the fact was established that the; sterility of sugar- cane under Natal conditions was not complete. The seed collected from the field almost certainly resulted from self-fertilization. Hybridizations were success- fully carried out for the first time in 1945. Unfor- tunately only a few of these hybrids came from crosses between commercial types of cane—most were from crosses in which the pollen parent was a Saccharum spontaneum, or "wild," form of sugar- cane. The major obstacle to sugarcane breeding in Natal—pollen sterility in commercial types of sugarcane—had still to be overcome.
In 1946 some success was obtained in inducing the variety Co.301 to produce fertile pollen. This was done by subjecting flowering shoots to artificial conditions which included night temperatures higher than the normal. However, several years were re- quired for the development of a really satisfactory technique for increasing pollen fertility, 2 3 4 and it was not until 1949 that reasonably large numbers of seedlings were being raised. Since then, improve- ments in technique and increasing experience have further extended the scope of sugarcane breeding in Natal. Recent findings have enabled the female fertility of many varieties to be increased by the use of the technique devised originally for increasing male fertility5 and have also made possible the im- proved, preservation of detached shoots used in pollen fertility studies. This better preservation results from the stimulation of rooting which follows the addition of methoxy ethyl mercuric chloride—
at the rate of 4.5 parts per million —to the standard sulphurous acid-phosphoric acid preserving .solution.
Though the annual production of .seedlings has tended to increase fairly rapidly, the advance has not
been altogether a steady one and fairly wide fluctua- tions have occurred from year to year. These fluctuations have for the most part been brought about by variations in. the intensity of tasselling.
A fairly good indication of the trend in seedling raising from J 944 to 1953 is given by the three-yearly averages presented below — in each case the year given is the central one of its period.
Vi'iu-
1945 1946 1947 1948 1949 1950 1951 1952
No. of Seedlings planted in the field (3-yoarly averages)
430 43(i 1,201 5,418 9,113 10,885 14,919 21,936 By 1953 the number of seedlings planted individu- ally in the field had reached 32,192; in addition, during this year about thirty thousand seedlings were planted in bunches.
Present Policy and Methods in Sugarcane Breeding
The aims of sugarcane breeders are the same throughout the world—the production of varieties yielding the maximum returns under the conditions for which they are intended—but different means are used in different places to attain the same end. The work of sugarcane breeding can be divided into two separate and distinct processes—the development from seed of new and superior varieties, and the separation of these superior varieties from the in- ferior ones which inevitably accompany them. Many variations of technique have been evolved in different parts of the world for handling these two processes.
The Production of Superior Types
The sugarcane breeding policies of most countries are based upon the use of trial crosses and are fundamentally similar. Usually a large number of trial crosses are made, and a relatively few seedlings of each planted in the field; the crosses found in this way to be the most promising are then repeated on as large as scale as possible. In Hawaii this system of breeding has been replaced to a great extent by one based upon the so-called "melting-pot" technique.
Economic reasons have been largely responsible for
this change, melting-pot crosses being easier to handle t h a n controlled crosses. Only the female parent is known of a seedling from a melting-pot cross, however, and if an outstanding seedling is obtained, the cross which produced it cannot be repeated. Obviously trial crosses cannot be used with this technique.
For the most part the making of controlled crosses presents fewer difficulties in Natal than in other countries. The advantage sugarcane breeding in Natal enjoys in this respect arises from what was once considered its greatest disadvantage—complete, pollen sterility of most tassels under field conditions.
This pollen sterility makes possible the extensive: use of area crosses—these, though controlled, require less supervision t h a n the usual bi-parental crosses. In making these crosses, tassels of one male variety, previously kept under controlled conditions to in- crease its pollen fertility, are used to fertilize the tassels of any number of female varieties taken from the. held. In tropical countries it often happens t h a t tassels which are not completely pollen-sterile have:
to be used as females. In such cases, if controlled crosses are to be made, only two varieties c a n be crossed at the same time—and even so, some self- fertilization may occur.
The making of area crosses in Natal is likely to be complicated by the development of the technique—
mentioned above—for increasing female fertility.
With this technique, pollen fertility is likely to be increased as well as ovule fertility. It is hoped a careful control of the temperatures to which the developing inflorescences are exposed, or of the dura- tion of the treatments, may enable ovule fertility to be stimulated without at the same time affecting pollen fertility. If no such method of control is found, some deviation from the present policy of making controlled crosses only m a y prove advisable.
The use of trial crosses as a breeding technique has sometimes been criticised on the basis t h a t a few hundred seedlings are unlikely to provide a proper indication of the full potentialities of a cross, espec- ially as it is the exceptional, rather than the average seedling of a cross, t h a t determines its value. This disadvantage is probably not in itself sufficient cause for abandoning the whole technique. It would seem reasonable to infer that a cross having a high, average, performance for its seedlings is more likely to produce seedlings of commercial standard than one having a.
low average performance—a correlation, not an absolute agreement, is to be expected between the averages a n d the extremes of crosses. In addition, in estimating the value of a trial cross t h e average performance of its seedlings need not provide t h e sole criterion—a few fairly promising t y p e s amongst a great many inferior ones can indicate t h a t the cross might have valuable potentialities, and be worth repeating.
For the reasons given above, it is intended to continue with the system of controlled pollinations and trial crosses as at present used in Natal. A record is being kept of the estimated value, of the different crosses at the single stool stage; this esti- mated value is recorded by the appropriate number on an arbitrary scale ranging from 0 to (i. At a. later stage, a. comparison of the estimated values of the various crosses and their actual values as shown by the performances of their seedlings should provide a useful check on the soundness of the technique at present in use.
It often happens t h a t only one of several possible crosses can be made, and certain generalisations may then he of help in deciding which cross should be attempted. For this reason records are kept for each variety of all the crosses in which it is used. The usefulness of these records in evaluating the breeding behaviour of varieties should increase with time. In the meantime it is necessary to avoid preconceived notions likely to restrict the range of crossing; at present the best rule would seem to be: that trying everything once- in so far as this is possible.
Seedling Selection The Criteria of Selection
The process of selection aims at a progressive elimination of less desirable seedlings until eventually only seedlings of commercial standard remain. The majority of seedlings are discarded because they lack vigour: obviously stunted seedlings are soon dis- carded; variety trials are necessary for eliminating the seedlings whose yields in sucrose per acre fall only slightly below those of standard varieties.
The next most frequent cause for eliminating seedlings is disease susceptibility.
Seedlings at the first stage of selection t h a t are found infected with the virus diseases of streak, mosaic, or chlorotic streak or with the fungus diseases of smut or red rot, are always discarded.
Characteristics regardeel as undesirable, but if not pronounced, as insufficient cause in themselves for discarding seedlings, include the following:
(1) thinness, pithiness or hollownc:ss of stalks;
(2) lodging or other type of inferior habit;
(•5) susceptibility to minor diseases;
(4) the presence of dead stalks in the seedling stools;
(5) sprouting of buds on growing canes;
(G) clinging of trash;
(7) hairiness of leaf sheaths;
(8) tasselling, especially during the. first season's growth.
The following are the likely changes in the propor- tions of the three main varieties of Natal. Co.301 has probably passed its peak and the area under it can be expected to decline. Both Co.331 and N:Co.
310, however, seem likely to increase in importance during the next few years—N:Co.3I0, of course, continuing to occupy the more extensive acreage.
Of the recently released varieties, N:Co.293 is adapted to the higher altitude regions; it will pro- bably increase rapidly in these regions until it covers quite an extensive area.. It is not likely to prove popular in the main coastal belt because of its tendency to flower very profusely, even during its first year's growth. The future of the variety N:Co.
339 is likely to depend upon its reaction to mosaic disease. It appears that N:Co.339, though retaining its tolerance to this disease.;, may have become more susceptible to infection. Nevertheless, the chances are that this high-yielding, versatile variety will prove of value in many parts of (he main coastal belt.
The variety N:Co.292, to be distributed soon, may find its niche within specialised areas of the cane belt, but it does not have the same chance of coming into general cultivation as the other N:Co. varieties.
Varieties at Present under Trial
Now that seedlings can be selected in Natal for their adaptation to local conditions, established varieties from overseas are less likely to qualify for general cultivation. The only imported variety at.
present in a variety trial is M.63/39. Future variety releases are most likely to come from seedlings raised in Natal—from either local or imported seed.
The position with regard to seedlings raised from introduced seed is now as follows. There is little likelihood that more commerical varieties—unless they are highly specialised—will be obtained from the particular batch of seed that, produced the varieties already released. This particular introduc- tion, however, was followed by many others—eight from Mauritius, two from Queensland, one from India and one from Hawaii.
The introduction from India consisted of a fudher supply of seed from three crosses which had pre- viously proved promising in Natal. The most promising varieties obtained from this introduction are N:Co.376 and N:Co.382. One or both of these varieties may later on qualify for release. N:Co.376 has the same parentage as the N:Co. varieties already released—that is, Co.421 x Co.31.2. N:Co.382 comes from the cross P.O.J.2725 X Co.301.
The seed introduced from Mauritius and Hawaii did not prove as valuable as t h a t from India. All the seedlings that were raised from this seed have now been discarded.
The seedlings raised from Queensland seed are still under trial. Only two of the first batch of seedlings now remain—they are in a variety trial. Fifty-three of the second batch are planted in single lines.
There is no chance of any varieties from crosses made locally being released in the immediate future.
An indication of the position at present reached in sugarcane breeding in Natal can be obtained from the following table:-
NUMBF.K OK SKKDI.INGS Year
Swelling (lerniinated
Planted as Single Stools
36 r>7-).
Selected for Selected for Single Lines Vaiiely Trials
1944 ... 36 14 0 1945 ... 572 30 7 MM 6 ... 681 58 5 19-17 ... 54 5 0 19-18 ... 2,868 175 1-1 1919 ... i 3,331 1,099
1950 ... 11,141. 580 1951. ... 8,183 251 >
1.952 ... 25,432
1953 ... 32/192* —
* An additional 30,000 seedlings were planted in bundle.;
As shown in the table, only twenty-six seedlings of local origin have reached the stage of being tested in a variety trial. Of the 1945 and .194(5 seedlings, only one out of twelve was selected for further testing. The 1948 seedlings were only recently planted in a variety trial and no results are as yet available. Up to the present, selections for variety trials have been made from relatively few seedlings.
Next season, however—and, one may reasonably hope, from then on—fairly large numbers of seedlings should be available for testing in variety trials. The seedlings which will be tested next season were raised in 1949—the first year that more than a. few thousand seedlings were obtained.
In conclusion, therefore, it may be said. that, although there is no immediate prospect of the release of any locally-produced seedlings, the large number at present under trial offers hope for the future.
1 Brett, P. G A frica. Nature,
REFERENCES C. (1040): Seed setting
157, ()f>7. of sugarcane in South
2 Brett, P. G. C. (104S): A possible method for increasing pollen fertility of sugarcane in Natal. S.A. Jour. Sci. 44, I2:J-1:24.
3 Brett, P. G. C. (1040): Further report on sugarcane breeding in South Africa. Proc. S.A. Sugar Tech. Assn., 23, I (IS-114.
4 Brett, P. G. C. (li)iiO): Investigations on sugarcane breeding in Natal during 1040. Proc. S.A. Sugar Tech. Assn., 24, 00-llCt.